1. Field of the Invention
The present invention relates generally to a microwave detector for detecting a microwave from a specific source, such as a measuring equipment. More specifically, the invention relates to a structure of a reception frequency converting portion for performing frequency mixing of a received wave by a horn antenna and an output of a local oscillator.
2. Description of the Related Art
Multi-band microwave detectors, such as so-called radar detectors which are adapted to detect microwaves transmitted from radar type speed measurement alarm equipment for alarming, have been known in the prior art. For example, U.S. Pat. No. 4,571,593, issued on Feb. 18, 1976 to Martinson, U.S. Pat. No. 4,939,521, issued on Jul. 3, 1990 to Burin, and U.S. Pat. No. 4,961,074, issued on Oct. 2, 1990 to Martinson disclose radar detectors of the type that the present invention is directed. Typical traffic monitoring radar type speed measuring equipment (hereinafter referred to as "radar") employ microwaves from among a plurality of frequency bands, i.e., 10 GHz band (X band), 24 GHz band (K band) and 35 GHz band (Ka band). The known radar detectors receive the microwave transmitted from the radar through a heterodyne type receiver circuit, such as a super-heterodyne receiver circuit.
In such a microwave detector, a horn antenna is generally employed as a reception antenna. At a feeding point in a throat portion of the horn antenna, a mixing diode is provided. A microwave circuit comprising a local oscillator is provided inside the throat portion of the horn antenna. The mixing diode performs frequency mixing of the reception signal received by the horn antenna and the output of the local oscillator. Also, in order to avoid affects of the reception input through the horn antenna for operation of the local oscillator, an appropriate filter is provided between the mixing stage and the local oscillator. In the known art, there is a type of construction wherein the mixer and the local oscillator are formed by a microwave circuit employing a wave guide. Also, it is known to form the mixer and the local oscillator with a microwave IC employing a micro-strip line.
Japanese Unexamined Patent Publication No. 61-500944 which corresponds to U.S. Pat. No. 4,571,593 discloses one of typical constructions of such a multi-band microwave detector. The structure disclosed in the above-identified publications will be briefly discussed herebelow with reference to FIG. 1.
In FIG. 1, a lower end opened aluminum die-casted antenna part 2 is fixedly secured on the upper surface of a printed circuit board 1 by screws. A metal layer (copper foil) 3 is formed on the upper surface of the printed circuit board 1 in a desired pattern. The antenna part 2 is mounted to cover the metal layer 3 in such a manner that the opened lower end of the antenna part 2 is press-fitted by the metal layer 3. The antenna part 2 is formed integrally with a horn antenna portion 2a and two shield casing portions 2b and 2c. The shield casing portion 2b is communicated with a throat portion of the horn antenna portion 2a.
The lower end opened horn antenna portion 2a cannot solely perform the function as the horn antenna but has to be cooperated with the metal layer 3 press-fitting the opened lower end for performing the desired horn antenna function.
A microwave IC circuit 4 is disposed in a space defined by the printed circuit board 1 and the shield casing portion 2b. The microwave IC circuit 4 includes a first local oscillator (not shown), a micro-strip line for guiding the output of the first local oscillator to a mixing diode 4b, a feed strip 4a kept in contact with a ridge 2d integrally formed at the center of the horn antenna portion 2a for receiving an antenna input, and a first intermediate frequency filter 4c for receiving a frequency mixing output obtained by mixing the antenna input with the output of the first local oscillator. These components of the microwave IC circuit are installed on a dielectric substrate.
At a portion of the printed circuit board 1 covered by the shield casing portion 2c, a second local oscillator, a second frequency mixing circuit for performing a frequency mixing of the output of the second local oscillator and the output of the microwave IC circuit 4, and a filter receiving the output of the second frequency mixing circuit are installed, although these components are omitted from illustration. A circuit composed of these elements will be hereafter referred to as "intermediate frequency processing circuit portion 5". A detector circuit for detecting the output of the intermediate frequency processing circuit, a discrimination circuit for discriminating a specific reception signal from the detection output, an alarm circuit for generating an alarm in response to the discrimination output and so forth are installed on the lower side surface of the printed circuit board 1.
As shown in FIG. 1, a construction employing the metal layer 3 formed on the printed circuit board 1 as a part of the constructional element of the horn antenna is advantageous from viewpoint of cost and from the viewpoint of down-sizing of the device in comparison with the construction employing the antenna part solely operable as the horn antenna. On the other hand, in the above-mentioned prior art, the first local oscillator, the first frequency mixing circuit, and the intermediate frequency filter are formed in the microwave IC circuit 4 to incorporate the input of the horn antenna as the microwave circuit with a wave guide in the feed strip 4a of the microwave IC circuit 4 as a microwave plane circuit. The microwave IC employing the micro-strip line is certainly beneficial from the viewpoints of down-sizing and making the circuit free from adjusting to gain required characteristics. However, it requires high precision in patterning of the strip conductor on a dielectric substrate and thus requires high designing skill. This results in disadvantages from the viewpoint of machining precision requirement and yield in production of the parts. In particular, for establishing desired relationship between the ridge 2d and the feed strip 4a for effectively feeding the antenna input, substantially high skill is required. To this respect, the microwave circuit employing the wave guide permits easy adjustment of the characteristics by aligning means, such as an iris, a pin or so forth, even after assembling the main body of the microwave circuit. Furthermore, know-how in the adjustment technology have been accumulated for many years. Accordingly, from the view of ease of adjustment, the microwave circuit with a wave guide is superior over the microwave plane circuit.
On the other hand, in the conventional construction of FIG. 1, since the intermediate frequency filter 4c is exposed to the antenna input and the first local oscillator output, the intermediate frequency filter may be affected for generation of the noise or so forth. Furthermore, similarly to the upper surface side of the intermediate frequency processing circuit portion 5 covered by the shield casing portion 2c, the lower surface side has to be shielded by a shielding metal layer or so forth. When the shielding metal layer is formed on the lower surface side of the intermediate frequency processing circuit portion 5, the lower surface side cannot be used for installation of other circuits. Accordingly, wider substrate area becomes necessary to increase the size of the substrate. Furthermore, it is cumbersome to provide a connecting means for transferring the output of the microwave IC circuit 4 to the intermediate frequency processing circuit portion 5 in the shield casing portion 2c. For instance, when a wiring pattern extending from the microwave IC circuit 4 to the intermediate frequency processing circuit portion 5 is formed on the printed circuit board 1, the wiring pattern and the antenna part 2 have to be insulated from each other. Also, for instance for establishing connection between the microwave IC circuit 4 and the intermediate frequency processing circuit portion 5, the surface side of the microwave IC circuit 4 is connected to the back side through the substrate by means of a pin, an eyelet and so forth, connected to the portion beneath the intermediate frequency processing circuit portion 5 by means of the wiring pattern on the back side of the substrate, and then connected to the intermediate frequency processing circuit portion 5 on the surface side of the substrate 1 via a pin, an eyelet, a through hole or so forth. Establishing such a connection is very cumber some.